1.Field of the Invention
The present invention relates to an optical receiver module and an optical receiver module system, and particularly relates to an optical receiver module and an optical receiver module system which are designed to remedy deterioration of a transmission characteristic (i.e. transmission penalty) in long-distance transmission.
2.Description of the Related Art
Regarding waveform distortion due to dispersion and polarization, reduction in transmission penalty is indispensable technique in a large-capacity long-distance optical transmission system because influence of the distortion becomes large with increase in distance in transmission and increase in bit rate.
Methods for suppressing the transmission penalty in the long-distance optical transmission include compensation of the waveform distortion using a variable dispersion compensator. The variable dispersion compensator gives an opposing dispersion level to an optical waveform having waveform distortion caused by the effect of fiber dispersion, and thus reproduces an original waveform before having the distortion. There is a method in which a code error information monitor circuit added to an optical receiver is added to monitor code error information at anytime, and thereby a dispersion value of the variable dispersion compensator is controlled such that code error due to waveform distortion after fiber transmission does not occur, so that the waveform distortion is suppressed, which is described in JP-A No. 2002-208892.
There is another method wherein a received signal is inputted into a band-pass filter and the variable dispersion compensator is controlled such that power output from the filter is minimized, which is described in JP-A No. 2004-254333.
In the case of the compensation of waveform distortion by the variable dispersion compensator using the code error information monitor circuit described in JP-A-2002-208892,an error correction circuit (FEC: Forward Error Correction) is used as the code error correction information monitor circuit. Since functions of an optical transceiver module is generally restricted to a function specialized to optic-electric signal conversion, the module does not have the FEC function. The FEC function can be realized by an LSI having the FEC function, which is mounted on a substrate for mounting the optical transceiver module on a system side. If the FEC function is provided on the optical transceiver module, it causes increase in size or increase in power consumption of the module. The FEC function is not necessary in applications in some system configurations, and in this case, the compensation of waveform distortion using the code error information monitor cannot be used.
The method using the band-pass filter described in JP-A No. 2004-254333 causes an increase in size, because the band-pass filter needs to be provided in the optical transceiver module. Moreover, the band-pass filter is generally expensive, leading to an increase in cost.
The method using the variable dispersion compensator for responding to the issue of reducing the transmission penalty cannot meet the demand for the optical transceiver module, including reduction in size, power consumption, and cost. Therefore, technique of suppressing deterioration of receiver sensitivity due to fiber dispersion is essential in a method without using the variable dispersion compensator. Again in this case, technique for preventing increase in size, power consumption, and cost of the optical transceiver module is indispensable.